In the legal battle against the Children's Internet Protection Act (CIPA), the government's position was to mandate filters on library computers as a way to protect children. The ALA and the ACLU argued that such filters were unconstitutional as they blocked speech protected by the first amendment, but also that the filters were ineffective to the purpose intended, letting some "inappropriate" material through. Judith Krug testified, saying: "Even the filtering manufacturers admit it is impossible to block all undesirable material." The government, of course, argued for filters.
Now, the Child Online Protection Act (COPA) from 1998 is going to court. This law requires that Internet sites that carry material that may be harmful to children use some method (such as requiring a credit card number) to prevent children from accessing the material, or face criminal charges if children access their site. In this case, the ACLU is expected to argue that filters are a better way to prevent children from see the offending material (having evolved since CIPA days), and the government will argue that filters are ineffective because a fair amount of pornography slips through them.
*sigh* It's the absurdity of it all that gets to me. That and my paranoid fear that it's all a plot to engage our limited resources while our rights erode on so many fronts.